The focus of this project is the use of E. coli to express the Chlamydia trachomatis major outer membrane protein (MOMP) for structural, functional and immunological analyses. MOMP induces protective immune responses and is believed to serve several important biological functions including acting as an adhesin. The immediate goals of this project are two-fold: first, to evaluate recombinant MOMP expressed in E. coli as an accurate and valid model for the native molecule and second, to express defined MOMP epitopes as fusions with immunologically stimulatory proteins. Expression of the MOMP gene (omp1) in E. coli resulted in signal peptide cleavage and the production of molecule with the correct molecular weight. The expression of this molecule was, however, extremely toxic to the host cells and neither surface localization nor the presence of conformational epitopes could be demonstrated. These results suggest that MOMP expressed in E. coli does not assume a native, surface localized, conformation and thus is of little utility as a structural or functional model; nor as a source for conformationally correct vaccine antigen. Defined B- and T-Cell MOMP epitopes have been expressed as fusions with either the A or B subunits of E. coli heat labile toxin LT. Chlamydia are primarily pathogens of mucosal surfaces and a successful vaccine will require the induction of a strong mucosal immune response. E. coli LT is a potent stimulator of mucosal immunity. MOMP/LT fusion may, therefore, be capable of stimulating strong anti-chlamydial immune responses. Genetic constructions which express MOMP/LT chimeras have been completed and characterization of the structural and functional properties of these molecules is ongoing.